It is appreciated that the density of the amorphous phase within biaxially-oriented polyethylene terephthalate (PET) film is directly related to the overall gas barrier properties. It is also appreciated that the amorphous content of any PET film is the weight fraction of material that is not crystallized within the biaxial orientation process, and that the densification of the amorphous phase implies that the mass per unit volume of non-crystalline material is increased (see, e.g., Polymer Bulletin, April 1988, Volume 19, Issue 4, pp. 397-401, which is incorporated herein by this reference in its entirety). In this regard, crystallinity and amorphous phase densification are both parameters that can be modified on a conventional biaxially-oriented stretching machine through stretch ratio conditions, the temperature of stretching, and the subsequent heat-setting of the film. Moreover, such densification is known to increase with the orientation of PET film.
In producing high-barrier PET films, however, although the use of an aluminum vacuum deposition process to produce a barrier layer in the film is common, the use of such a process does not guarantee the suitability of the resulting films for very high-barrier requirements due to the potential for defects. Indeed, defect population and type of defect become limiting factors for very high-barrier applications making use of vacuum-deposited metal surfaces, such as what may be found in the high-barrier films that are often necessary for extending shelf-life in food and electronics packaging, tobacco, medical packaging, and other industrial uses such as the fabrication of balloons.
Vacuum metallizing thin films at the high optical densities, often greater than 2.7 OD, needed for very high gas barrier applications can result in thermal distortions to the films during the process. Indeed, it is typical for these distortion problems to result in heat lanes or tracks or, in other words, areas of localized shrinkage within the PET film structure that are due to a high heat load that is not dissipated quickly. Metal adhesion, barrier properties, and appearance are then all poor within the film areas that heat lanes develop.
In order to combat the defects caused by heat lanes, it is common to use a gas wedge in the vacuum metallizing process. This process consists of injecting a gas between the moving PET web and the chill roll in the vacuum metallizing process. The increased thermal transfer from chill roll to film is often enough to reduce heat lanes in metallized films to an acceptable level. However, in the case of very thin films (less than about 10 μm), the ability of the film to traverse the chill roll assembly without sticking, wrinkling, or distorting is a serious issue. Additionally, in such situations, the use of high tensions on the thin film to keep the film flat on the chill roll is practically impossible due to the tensile limitations of the film under a heat load. One solution to this problem is to run the film at reduced speeds in the metallizing chamber. However, the significant commercial cost of running film at reduced speeds makes that approach unrealistic.
Accordingly, there remains a need in the art to produce a thin, high-barrier, and high-adhesion PET film at commercially-acceptable line speeds. Such films and processes are particularly desirable and beneficial for a range of applications in food and consumer packaging and in the fabrication of industrial commodities.